JIG DESIGN 9
locate or change the bushings. When designing the jig, these points should be remembered and provision made for them, where possible.
General Remarks on Jig Design. — One mistake, quite frequently made, is that of giving too little clearance between the piece to be machined and the walls or sides of the jig used for it. Plenty of clearance should always be allowed, particularly when rough castings are being drilled or machined in the jigs; besides, those surfaces in the jig which do not actually bear upon the work do not always come exactly to the dimensions indicated on the drawing, particularly in a cast-iron jig, and allowance ought to be made for such differences.
In regard to the locating points, it ought to be remarked that, in all instances, these should be visible to the operator when placing the work in position, so that he may be enabled to see that the work really is in its right place. At times the construction of the piece to be worked upon may prevent a full view of the locating points. In such a case a cored or drilled hole in the jig, near the locating seat, will enable a view of same, so that the operator may either see that the work rests upon the locating point, or so that he can place a feeler or thickness gage between the work and the locating surface, to make sure that he has the work in its correct position. Another point that should not be overlooked is that jigs and fixtures should be designed with a view of making them easily cleaned from the chips, and provision should also be made so that the chips, as far as possible, may fall out of the jig and not accumulate on or about the locating points, where they are liable to throw the work out of its correct position and consequently spoil the piece.
The principles so far referred to have all been in relation to the holding of the work in the jig, and the general design of the jig for producing accurate work. Provisions, however, should also be made for clamping the jig or fixture to the table of the machine, in cases where it is necessary to have the tool fixed while in operation. Small drilling jigs are not clamped to the table, but boring jigs and milling and planing fixtures invariably must be firmly secured to the machine on which they are used.